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Soft Brake Pedal Issues


rattytat

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i have 930 calipers fitted to a competition car.  Balance pedal box fitted so able to balance between front and rear brakes.  Two separate circuits (front and rear) and racing brake fluid, brakes bled several times - no air in the system and no fluid leaks.  Brake pedal goes well down and is soft - pedal effort doesn’t change even when pumped…. Nothing like the brakes on the road going 930.  What am I missing - any thoughts?? Thanks, Dave 

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Also, did you bleed the master cylinder? This can also help  - sometimes just bleeding the system at the calipers doesn't completely clear air at the calipers. I had exactly this on my SC, albeit with a more or less standard set up. You can bleed the master cylinder independently while it's still in the car by using some barbed fittings and plastic tubing routed back into the reservoir and a handy assistant operating the pedal. I found that even after a full standard bleed, I still had some bubbles coming through using this method after 10 minutes of pumping the pedal.

May not be your issue but well worth a try.

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13 hours ago, rattytat said:

Balance pedal box fitted so able to balance between front and rear brakes.  Two separate circuits (front and rear) and racing brake fluid, brakes bled several times - no air in the system and no fluid leaks.  Brake pedal goes well down and is soft - pedal effort doesn’t change even when pumped….

Sounds like the size on one, or both, of the master cylinders is too small for the calipers.

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5 hours ago, Ian Comerford said:

What master cylinder do you have fitted, this can affect pedal effort/feel?

Not sure Ian - I’ll check

5 hours ago, Phill said:

How recently did you put this together and fit the pads?

The system has been together for a while Phill, brakes bled fairly recently for the nth time…

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6 hours ago, rattytat said:

Not sure Ian - I’ll check

The system has been together for a while Phill, brakes bled fairly recently for the nth time…

The reason for asking is that Lewis recently swapped to Boxster calipers on his SC and noticed that the pedal firmed up a lot when he fitted a Turbo/930 master cylinder.  Maybe your MC isn’t big enough.

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22 hours ago, Lesworth said:

Also, did you bleed the master cylinder? This can also help  - sometimes just bleeding the system at the calipers doesn't completely clear air at the calipers. I had exactly this on my SC, albeit with a more or less standard set up. You can bleed the master cylinder independently while it's still in the car by using some barbed fittings and plastic tubing routed back into the reservoir and a handy assistant operating the pedal. I found that even after a full standard bleed, I still had some bubbles coming through using this method after 10 minutes of pumping the pedal.

May not be your issue but well worth a try.

My thoughts are master cylinder related, somehow.  Not sure whether it’s size or wear/damage related or air but I’m conscious that pumping the pedal doesn’t make any difference to its firmness.  I can find no leaks, the lines are all new and the calipers are working, it’s just that the pedal travels a fair distance before it’s starts to build pressure….

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Do you have one or two master cylinders?  
As said above the turbo MC is the preferred upgrade when fitting bigger brakes.   
Problem is that Turbo MCs are hugely expensive at the moment so you may find a different option better.

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7 hours ago, Phill said:

Do you have one or two master cylinders?  
As said above the turbo MC is the preferred upgrade when fitting bigger brakes.   
Problem is that Turbo MCs are hugely expensive at the moment so you may find a different option better.

Two separate master cylinders Phill, so the front and rear brakes are on separate circuits per the 911 SCRS set up of the mid 80’s 🤔

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1 minute ago, rattytat said:

Two separate master cylinders Phill, so the front and rear brakes are on separate circuits per the 911 SCRS set up of the mid 80’s 🤔

Aha, so you should be able to easily (and relatively cheaply) swap them out for bigger ones or at least the one for the front circuit?

Got any pics - interested.

Edited by Phill
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21 minutes ago, Phill said:

Aha, so you should be able to easily (and relatively cheaply) swap them out for bigger ones or at least the one for the front circuit?

Got any pics - interested.

Yes, I think so…  I’ll put more up and the story when I get a little bit further…

 

 

IMG_1904.jpeg

IMG_1900.jpeg

IMG_1899.jpeg

IMG_1902.jpeg

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14 minutes ago, Phill said:

That looks great. I meant pictures of the brake set up but anything about this car will be good :signs85:

🙄😂, sorry Phill my mistake - I’ll get some and post them up 👍

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  • 3 weeks later...
On 14/05/2024 at 06:48, Ian Comerford said:

Did you get anywhere with this?

Not yet Ian.  I’m going to get the pedal box out and take a closer look, I’m thinking it’s either size of the master cylinders, or possibly even a damaged seal 🤔

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Any chance you can post a photo of the base of the brake pedal where the bias bar is located please?  There could be an issue with the way it is set up, but I’ll need to see a photo to check.  I can give you more explanation of that when I see the photograph, but essentially if there aren’t anti-foul washers in place you could have an issue.

Secondly, when you are bleeding are you bleeding front and rear at the same time?  This is the proper way to bleed a bias box, because bleeding front or rear only at any given time means you might not be achieving full travel on the cylinder.  Common issue and easily cured.

Regarding master cylinder sizing it could be the case that the cylinders are small and in the 0.625” / 0.70” range and if so, you could have a lot of travel and a high line pressure.  Increasing the cylinder sizes could improve things, but not necessarily.  You could end up with a solid pedal that has no travel, but that you can’t press hard enough to make a meaningful stop.  It is a bit of a black art.

As a general rule of thumb, with the bias bar set in the middle of the travel, assembled correctly and the brakes bled up properly, you are looking for three turns either side of centre to achieve the correct bias in my experience.  If you can’t get correct bias within that, it is time to swap cylinders, but keep the differential between them to one size difference; 0.625” and 0.70” or 0.70” and 0.75”, as examples.  If you are using AP stubbies, there are some funky sizes that sit between the ‘traditional’ sizes, which allow for fine tuning, but none are cheap.

Finally and presuming it is an aftermarket pedal or pedal box, what make is it?  Also, what make pad and compound are you using?

 

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8 hours ago, oliverjamesthomas said:

Any chance you can post a photo of the base of the brake pedal where the bias bar is located please?  There could be an issue with the way it is set up, but I’ll need to see a photo to check.  I can give you more explanation of that when I see the photograph, but essentially if there aren’t anti-foul washers in place you could have an issue.

Secondly, when you are bleeding are you bleeding front and rear at the same time?  This is the proper way to bleed a bias box, because bleeding front or rear only at any given time means you might not be achieving full travel on the cylinder.  Common issue and easily cured.

Regarding master cylinder sizing it could be the case that the cylinders are small and in the 0.625” / 0.70” range and if so, you could have a lot of travel and a high line pressure.  Increasing the cylinder sizes could improve things, but not necessarily.  You could end up with a solid pedal that has no travel, but that you can’t press hard enough to make a meaningful stop.  It is a bit of a black art.

As a general rule of thumb, with the bias bar set in the middle of the travel, assembled correctly and the brakes bled up properly, you are looking for three turns either side of centre to achieve the correct bias in my experience.  If you can’t get correct bias within that, it is time to swap cylinders, but keep the differential between them to one size difference; 0.625” and 0.70” or 0.70” and 0.75”, as examples.  If you are using AP stubbies, there are some funky sizes that sit between the ‘traditional’ sizes, which allow for fine tuning, but none are cheap.

Finally and presuming it is an aftermarket pedal or pedal box, what make is it?  Also, what make pad and compound are you using?

 

Excellent advice and knowledge :signs85:

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On 20/05/2024 at 21:56, oliverjamesthomas said:

Any chance you can post a photo of the base of the brake pedal where the bias bar is located please?  There could be an issue with the way it is set up, but I’ll need to see a photo to check.  I can give you more explanation of that when I see the photograph, but essentially if there aren’t anti-foul washers in place you could have an issue.

Secondly, when you are bleeding are you bleeding front and rear at the same time?  This is the proper way to bleed a bias box, because bleeding front or rear only at any given time means you might not be achieving full travel on the cylinder.  Common issue and easily cured.

Regarding master cylinder sizing it could be the case that the cylinders are small and in the 0.625” / 0.70” range and if so, you could have a lot of travel and a high line pressure.  Increasing the cylinder sizes could improve things, but not necessarily.  You could end up with a solid pedal that has no travel, but that you can’t press hard enough to make a meaningful stop.  It is a bit of a black art.

As a general rule of thumb, with the bias bar set in the middle of the travel, assembled correctly and the brakes bled up properly, you are looking for three turns either side of centre to achieve the correct bias in my experience.  If you can’t get correct bias within that, it is time to swap cylinders, but keep the differential between them to one size difference; 0.625” and 0.70” or 0.70” and 0.75”, as examples.  If you are using AP stubbies, there are some funky sizes that sit between the ‘traditional’ sizes, which allow for fine tuning, but none are cheap.

Finally and presuming it is an aftermarket pedal or pedal box, what make is it?  Also, what make pad and compound are you using?

 

Ho Oliver, apologies only just seen your post - but many thanks for your post, helpful comments and effort in the response - leave it with me and I’ll get some photos posted, and get back to you.

Thanks again, Dave

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  • 5 weeks later...

Gents, apologies for the delay in posting these photos but as promised, best photos I have of the pedal box for comments...

 

 

911 pedal box 1.jpg

911 pedal box 2.jpg

911 pedal box 3.jpg

911 pedal box 4.jpg

911 pedal box 5.jpg

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How far do the braided lines run from the MCs? Do they go all the way to the brakes? How long have they been on there?

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A few things strike me that need a bit of rectification, unless it is a trick of the lens.  A bit of disassembly is required in order to get the foundations right.

Are the pushrods sitting parallel to each other and in a straight line between the clevis and the master cylinder?  The one on the right looks to be sitting at an angle, possibly because the tube holding the bias bar is offset to the right?  The pushrods need to be actioning in a straight line during pedal travel, so I think the excess material will need removing from the right of the bias bar tube.

There are no washers sitting between the clevis and the bias bar, meaning there is a possibility of the clevis fouling the bar during pedal movement and you don’t get full travel on the pushrod.  Fit a couple of oversize washers between each clevis and respective side of the bias bar tube, leaving a millimetre or so of space so that things aren’t bound up and the bias bar can turn freely.

The bias bar looks to be significantly away from a central position.  On reassembly, wind it all the way to one side and then all the way to the opposite, counting the number of turns.  Then wind it back half way.  That is your starting point.  My experience in use is that three turns each side of centre is okay, but any more would indicate the cylinder sizes are incorrect and you need to change them.  The picture suggests it is would fully to the right, unless I’m looking at it wrong.

The bias bar looks to be sitting straight at rest, which is unlikely to be right.  With everything set up correctly and the system bled up, the bar would generally sit at an angle because the front brakes will require more stroke than the rear.  With reasonable pressure applied to the pedal, the bias bar should be parallel with the bulkhead.  All this is adjusted with the system bled, by winding the pushrods in or out of their respective clevis.  I’ve seen loads of bias bars installed that are parallel with the bulkhead at rest, but have yet to see one where this is correct.  It may be possible in some circumstances, but unlikely in my opinion.

When you bleed the system up, make sure to bleed a front and rear caliper at the same time.  This is often a point of discussion in the pits, but I can safely say those that do are right and those that don’t are wrong 🤣

Have you got anything else in the system, like a secondary pressure valve, a line lock or a hydraulic handbrake?  They can all be problematic because they end up with an air pocket that you can’t get rid of.  Equally, don’t be scared to jack the front or rear of the car up when you bleed the system up.  I ran a car a couple of years ago that had Sierra rear calipers which trapped air because the bleed nipple didn’t quite sit at the top of the body.  I ended up unbolting the caliper and bleeding up with a spanner between the pads instead of the disc, so I could orientate the bleed nipple to the highest point.  It was a pain, but cured the problem and the pedal feel improved massively.

A final thought, but not a contributor to your woes; I’m a fussy bugger 🤣 I really dislike banjo fittings.  If you start changing master cylinders, fit them with a male-male adapter and put a AN-3 fitting on the end of the pipe in place of the banjo fitting.  The same goes for calipers as well; get rid of the banjo’s.  Repeatedly screwing a banjo into a thread is never a good thing and the are entirely too many stories of seized banjos or damaged threads.  The same is happening with bleed nipples as well; there is a relatively young GT3 kicking about at the minute where the garage is quoting silly money for a new caliper because the bleed nipple is seized and it isn’t covered as a warranty item.

Hope the above is of some use.  If you need some hands on help and let me know where you are in the Country, I might know someone I can point you in the direction of who can assist 👍

 

 

 

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